Gale Warning 257, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 AM AKDT Sat Mar 16 2019

.ANALYSIS AND UPPER LEVELS...

A shortwave in the northern Bering Sea is moving southeastward
toward the eastern Aleutians this morning. This shortwave has
little upper level support as the jet stream remains meridional
and well south of the Aleutian chain at 30,000 ft. The main
weather maker today is a vertically stacked low that sits just
south of Kodiak Island. Cold air is wrapping around the backside
of the low, pushing over the Bering and Aleutian Chain. In
contrast, southerly flow from the Pacific is ushering in warm and
moist air over the Gulf of Alaska and Southcentral.

At the surface, a warm front is butting up against the northern
Gulf Coast and is draped over the western Gulf. In the northern
Gulf, a barrier jet has formed with east-southeasterly winds.
Through the marine waters, winds up to 50 kts are being reported
in the northern Gulf in areas around Middleton Island and Prince
William Sound. Gusty conditions continue through the typical
favored passes, northern coastline areas like Cordova, and
Turnagain Arm. Precipitation (mainly in the form of rain) is
widespread in coastal areas while areas west of the Chugach
mountains are dry due to downsloping. Radar imagery also depicts
precipitation falling through Bristol Bay, the AK-Pen and
Kuskokwim Delta along a deformation band.

&&

.MODEL DISCUSSION...Models remain in rather good agreement with
just about all major features through the weekend. The result in
moderate to high forecast confidence. The few detail items we are
tracking are the exact location of the deformation axis over
Southwest AK and the exact height of the snow levels across the
Eastern Kenai Peninsula/Western Prince William Sound. Generally,
the NAMNest was preferred with this package for higher resolution
details.

&&

.AVIATION...
PANC... Weak northwest to west winds at the surface will allow for
wind shear to persist this morning. Southeasterly winds will
develop at all levels this afternoon as the warm front in the
northern Gulf moves inland. Timing of the wind shift should be
close to 00z or slightly thereafter. Downsloping will keep the
airport VFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today and Sunday)...

The active pattern continues as a series of surface lows and
fronts drive north across the Gulf. The first of the fronts will
make landfall over the North Gulf Coast this morning then move
inland this afternoon. Storm force easterly winds over marine
waters will weaken through this morning. Strong gap flows from
Portage Valley to western Turnagain Arm will peak this morning.
Pressure rises on the backside of the front as it moves inland
this afternoon will give rise to gusty southerly winds across the
Anchorage Bowl, Knik Valley, and over the Copper River Basin.
These winds should slowly diminish overnight tonight. Turnagain
Arm to Portage Valley winds will remain fairly strong and gusty
and perhaps see an uptick this evening as the surface low heads
north over the western Gulf. The next front heading north Sunday
will help to keep the Turnagain Arm winds going, as well as,
strengthening marine winds along the North Gulf coast.

As with the previous frontal systems, the brunt of the
precipitation will be confined to the Gulf coasts and should be
generally rain with snow levels in the 1800-2500 ft range through
tonight. Strong cross-barrier flow over the coastal mountains
will mitigate precipitation inland. The exception would be
potential for rain and snow for the Southern Copper River Basin
with next front on Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The area will be split into 2 regimes for much of the weekend.
Over the eastern half, southwest winds associated with lows in the
Gulf of Alaska will push through the AK and Aleutian Ranges. These
winds will also aid in ushering in some warm air and plenty of
moisture. 850mb (5,000') temperatures will climb to near 0 degrees
Celsius near places like Iliamna through the day. This will cause
precipitation that does make it to fall as mostly rain. Now it
will be a different story over the Western Capes and much of the
YK-Delta where cold air is intruding from the Bering. A fairly
vigorous shortwave over the Eastern Bering will push in plenty of
cold air with west to northwest winds. In between these two
systems, a deformation axis is taking shape. This will be the key
feature for weather impacts through the weekend as we expect
accumulation snow to accompany it. The band will shift west early
today and center near Bethel. But by this evening, it will drift
back to the east near Aniak and Sleetmute. Several inches of snow
accumulation are possible depending on where the band lingers the
longest, but we do expect amounts to remain below advisory level.

On Sun, the shortwave from the Bering pushes further inland over
SW AK. The net effect will be to drive cold and drier air through
much of the area. But as it gets over Port Alsworth and Iliamna,
it will run into the warm, moist air from the Gulf of AK. This
will lead to another potential snow set up for these far interior
locations. We will be monitoring this portion of the forecast for
accumulating snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
Sun)... The Bering Sea and Aleutians remain extremely quiet
during the weekend. This morning, northwest winds will remain
gusty through the Eastern Aleutians and Alaska Peninsula (Pacific
side marine zones) with cold air advection pushing through the
terrain. One small area of low pressure will swoop through from
the NW Bering this morning into the Pribilof Islands by this
evening before eventually falling apart as it enters Bristol Bay
Sun afternoon. This system will bring a pocket of small craft
winds with the possibility of a few gale- force gusts. Otherwise,
flow will generally be out of the northwest with seasonable cold
air helping to create pockets of snow showers.

&&

.MARINE (Days 3 through 5/Sunday through Wednesday)...

Current weather in both the Bering Sea/Aleutians and Gulf of
Alaska will remain pretty much the same. Gale force winds will
slowly taper off in the coastal waters outside Prince William Sound.
The Bering will remain pretty quiet. While models are still not
in great agreement, with this weather pattern we are confident the
forecast will pan out as expected.

&&

.LONG TERM FORECAST (Days 3 through 7/Sunday through Friday)...

The weather pattern for the long term sees little shift with
continued troughing over the Bering Sea and southerly flow through
Southcentral AK. This means continued weak flow and quiet weather
for the Bering Sea; Southcentral AK will continue to receive low
pressure systems every few days. As a result, the warm and wet
pattern will be sustained over southcentral AK for the foreseeable
future. Models hint at a larger low developing in the Gulf of
Alaska late next week. However, there is little agreement by this
time in the long term so we will have to wait and assess over the
next few days.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119.
         Gale Warning 351 352 120 125 130 131 139.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/MSO
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
MARINE/LONG TERM...BB